When a network of nodes share a medium it is necessary to allocate medium resources between the nodes. When the number of nodes is relatively small, conventional allocation methods may work acceptably. However, with a network with many nodes conventional methods do not work well. One conventional method of reserving medium access is defined by the IEEE standard 802.11 Medium Access Control (MAC) specification. In that MAC specification, when a node wishes to transfer a packet of information it sends out a request to send (RTS). When the base station sees this, it sends out a clear to send (CTS). If a second node wishes to transfer a packet, it also sends an RTS. However, it will not get a CTS until the first node gets its CTS. Unfortunately, this linear technique is unsuitable for a network with a large number of nodes.
For example, if there are 1024 nodes in the network all making reservations on the shared medium using the standard IEEE 802.11 MAC protocol, the reservation protocol overhead will be unacceptable. The total time between the beginning of the RTS and the end of the CTS may be 50 microseconds. For example, if the RTS packet contains 100 bits, which are transferred at 10 Mbps, then the packet duration will be 10 microseconds. Additionally, each of the following may be 10 microseconds: CTS packet duration, propagation delay each way, and processing time. Thus, with approximately 1000 nodes, the total periodicity for the reservation polling is 50 ms, which is the minimum latency for any packet. Latency may be even higher due to other delays. Consequently, the overhead for reservation is 100 percent: 10 Mbps for the MAC protocol function and 10 Mbps for the data link that carries the data buffered over the 50 ms duration for all nodes participating.
Additionally, IEEE std. 802.11 MAC protocol does not support Synchronous Channel (SCH) operation. However, some networks carry voice and video traffic, which require such operation. Consequently, IEEE std. 802.11 is unsuitable for such networks.
Another conventional method of allocating medium resources is the Data Over Cable Service Interface Specification (DOCSIS). In this technique, downstream traffic is time-slotted; however, upstream traffic is random access. If a collision occurs when nodes are requesting an upstream transfer, then the nodes perform a backoff and retry. The backoff time is specified to increase as the frequency of collisions increases. Thus, with a large number of nodes placing requests, the backoff time may become very long. This conventional method works best for systems that have mostly downstream traffic and little upstream traffic. Additionally, it does not work well for synchronous transfers, and is thus not suitable for voice applications.
Thus, a need has arisen for a method for allocating resources in a large distributed network, which share a medium. A further need exists for a method that does not have a high overhead. A still further need exists for such a method that works well for a network with considerable upstream traffic. An even further need exists for such a method that, while providing for asynchronous medium reservations, allows for synchronous transfers as well.